Add missing break

Found by GCC 7
 qwaylandinputmethodeventbuilder.cpp:110:51: warning: this statement may fall through [-Wimplicit-fallthrough=]

Change-Id: Ia3e896da908f42939148fffd14c47fc6058b0933
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Johan Helsing <johan.helsing@qt.io>

src/plugins/platforms/wayland/shared/qwaylandinputmethodeventbuilder.cpp

@@ -108,6 +108,7 @@ void QWaylandInputMethodEventBuilder::addPreeditStyling(uint32_t index, uint32_t
         format.setFontUnderline(true);
         format.setUnderlineStyle(QTextCharFormat::SingleUnderline);
         m_preeditStyles.append(QInputMethodEvent::Attribute(QInputMethodEvent::TextFormat, index, length, format));
+        break;
     case 5:
         format.setFontUnderline(true);
         format.setUnderlineStyle(QTextCharFormat::WaveUnderline);